2 Profs. J. Dewar and J. A. Fleming. On the Dielectric 



t; Note on the Dielectric Constant of Ice and Alcohol at very 

 Low Temperatures." By James Dewar, M.A., LL.D., 

 F.R.S., Fullerian Professor of Chemistry in the Royal 

 Institution, London, and J. A. Fleming, M.A., D.Sc, 

 F.E.S., Professor of Electrical Engineering in University 

 College, London. Received January 27, — Read February 

 25, 1897. 



Of late years many careful determinations have been made of the 

 dielectric constants of water and ice by different observers. These 

 evaluations may be divided into two classes. Firstly, those which 

 are, strictly speaking, determinations of the specific inductive 

 capacity of the material, and have been made by measuring the 

 change in the capacity of a condenser when water or ice is substi- 

 tuted for air as the dielectric. Secondly, those which are really 

 measurements of the electrical refractive index of water or ice for 

 electric waves of various lengths, and which have been generally 

 made by obtaining the reduction in wave-length experienced by an 

 electric wave on passing from air into water or ice. The square of 

 this refractive index or the ratio of wave-length reduction is then 

 taken as the dielectric constant. 



In order that results thus recently obtained may be compared, we 

 have collected into two tables (I and II) some of the values for the 

 specific inductive capacity, and the electric refractive index of water 

 for waves of various lengths. 



The determinations of the specific inductive capacity quoted all 

 appear to have been made by methods which, whilst excluding, or 

 believed to exclude, error arising from the conductivity of the water, 

 may yet be regarded as giving the value corresponding to com- 

 paratively slow reversals of electromotive force or to waves of infinite 

 wave-length. The electric refractive index observations have been 

 made by using electric waves of lengths in air varying from 8 mm. 

 to 600 cm. 



Two very careful determinations of the specific inductive capacity 

 of water seem to be those of W. Nernst (80'0 at 17° C.) and F. 

 Heerwagen (= 80'88 at 17° C), and that of J. IJ. Smale is in close 

 agreement with that of Nernst. 



As regards the electrical refractive index, it will be seen that the 

 determinations of P. Drude for waves of 70 cm., which give p, = 

 8-95, and hence ^ = 80'2 at 17° C, and those of Cohn and Zee- 

 man, which give /u, — 8*91 and /r = 79*39, as a mean value for 

 waves from 155 to 560 cm. in length, are in fairly close agreement 

 with one anotner, and with the best determinations of specific induc- 

 tive capacity. 



